Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define those millions of magnetic tunneling junction (MTJ) cells in each MRAM device and make them non-interacting to each other, precise patterning steps including RIE (reactive ion etching) are usually involved. During RIE, high energy ions remove materials vertically in those areas not masked by photoresist, separating one MTJ cell from another. However, the high energy ions can also react with the non-removed materials, oxygen, moisture and other chemicals laterally, causing sidewall damage and lowered device performance.
To solve this issue, pure physical etching techniques such as ion beam etching (IBE) have been applied to trim the surface of the MTJ stack to remove the damaged portion. However, due to their non-volatile nature, IBE trimmed conductive materials in the MTJ and bottom electrode can be re-deposited onto the tunnel barrier, resulting in shorted devices. A new device structure and associated process flow which can reduce this conductive material re-deposition are needed if one wants to fully utilize the benefits of physical etching.
Several patents teach methods of forming the bottom electrode to MTJ connection while avoiding re-deposition of metal on the MTJ sidewalls, including U.S. Pat. No. 8,324,698 (Zhong et al) and U.S. Pat. No. 8,883,520 (Satoh et al). These references are different from the present disclosure.